The structural features of the integrated modular small nuclear reactor are different from the traditional nuclear reactor. Besides the core barrel assembly, there are core support assembly, upper core support assembly, and flow distribution device in the pressure vessel of the integrated modular small nuclear reactor. The modal characteristics of the integrated modular small nuclear reactor internals is very important. In this paper, experimental and numerical studies were carried out to obtain the natural frequencies and modes in air and water, respectively. The comparisons of the modal characteristics of the reactor internals both in air and water between the experimental and numerical results illustrated that the analysis method can be used to predict the natural frequencies and modes.